There has been known a focused ion beam system as a system for performing observation, various evaluation or analysis, or the like on a sample such as a semiconductor device, and for preparing a TEM sample by taking a fine and thin sample fragment from a sample and fixing the thin sample fragment to a sample holder.
This focused ion beam system includes an ion source for generating ions and accelerates the ions generated in the ion source to generate an ion beam, and focuses the ion beam to radiate the ion beam as a focused ion beam (FIB).
There are many types of ion sources. In many cases, a focused ion beam system having been put to practical use includes a liquid-metal ion source (particularly, liquid-gallium ion source). However, a transmittance of a sample such as a photomask may be reduced due to injection of gallium, and therefore, countermeasures such as low acceleration have been employed. Incidentally, there is a need for performing observation on a sample having a nanometer size with low damages, and thus it is required to further reduce a diameter of the beam.
Accordingly, in recent years, there has been employed a gas field ion source (GFIS) as a non-contaminating ion source. The gas field ion source uses noble gases as ion species and is capable of generating ion beams having a smaller diameter and higher luminance as compared to liquid-metal ion sources. It has been confirmed that the gas field ion source has certain effects against the above-described problem.
A gas field ion source includes an aciculate emitter with a tip which is sharpened at an atomic level. This emitter is an important member for ionizing a gas, and thus the surface structure of the emitter is important. In order to achieve a high-luminance ion source, it is necessary to make the tip of the emitter as sharp as possible and arrange the shape of the tip such that an ionization area is formed by several atoms. Accordingly, it is possible to locally ionize a gas into gas ions, and thus it is possible to generate an ion beam having a small beam diameter.
There have been known some methods of sharpening a tip of an emitter. As one of those methods, there has been known a field-assisted gas etching method of locally etching a tip of an emitter while supplying oxygen or nitrogen to the vicinity of the emitter (see, for example, US2007/0025907A).
According to this method, it is possible to acquire an emission pattern image (i.e. field ion microscope (FIM) image) of the tip of the emitter and grasp the progress status of etching while observing the FIM (FIM observation), and thus it becomes easy to perform sharpening, so that this method is regarded as a promising method.
In order to acquire an FIM image, a device including a micro-channel plate (MCP) which has a fluorescent surface at a back side and is disposed on a light path of an ion beam is generally used. This device can be incorporated in a beam lens barrel, and can amplify a received ion beam (or focused ion beam) by the MCP and make the amplified beam enter the fluorescent surface, thereby projecting an FIM image onto the fluorescent surface. Accordingly, FIM observation on the tip of the emitter is performed.
In addition to the above-described MCP method, there has been known a method of performing FIM observation on a tip of an emitter by a scanning-FIM (see, for example, JP-A-2012-98293).
According to this method, an axis alignment deflector is used to perform raster scanning on a diaphragm surface, thereby capable of acquiring a field emission pattern image of a tip of an emitter. Accordingly, FIM observation on the tip of the emitter is performed.
Incidentally, when performing field-assisted gas etching, in order to accurately grasp the progress status of etching on a tip of an emitter, it is necessary to confirm the tip of the emitter over a wide visual field. Specifically, the emitter after electric field polishing needs to be confirmed at a visible field of about several tens of atoms or more.
Further, when incorporating the device including the MCP in a focused ion beam system, it is desired to dispose a condenser lens (a focusing lens electrode) 101 close to an emitter 100 which is an ion source, and then dispose an MCP 102 immediately below the condenser lens 101, as shown in FIG. 12. This is because it is necessary to reduce a distance between the emitter 100 and the condenser lens 101 for securing the performance of the focused ion beam system.
However, if the emitter 100 and the condenser lens 101 are disposed close to each other, since a structure such as the condenser lens 101 is disposed immediately above MCP 102, the peripheral component (oblique portions shown in FIG. 12) of an ion beam 103 may be blocked by the structure, so that the amount of ion beam 103 incident on the MCP 102 would be reduced. Therefore, the visual field would narrow, and it would be difficult to confirm the tip of the emitter 100 over a wide range.
Additionally, if the MCP 102 is used to observe an FIM image for a long time, the multiplication factor of the MCP 102 or the fluorescent surface may be deteriorated, so that the FIM image may become darker. Therefore, it may become difficult to clearly observe the tip of the emitter 100, and it may become difficult to accurately perform FIM observation.
Meanwhile, according to the method using the scanning-FIM disclosed in JP-A-2012-98293, since no MCP is used, even if an FIM image is observed for a long time, the FIM image does not become dark. However, even in the scanning-FIM, similarly, a visual field may be restricted by a structure such as a condenser lens.